San Diego is generally pretty low humidity. Low humidity generally does not allow anthracnose to get going. I am skeptical of anthracnose being the correct diagnosis. I'm not saying impossible, it could be anthracnose, but I think much more likely are the abiotic causes: desert heat, and bad water.
Japanese maples come from humid forests in a very mild, temperate climate that has enough rain that parts of the native range for Japanese maples is considered a temperate rain forest.
San Diego is the opposite of what a Japanese maple can tolerate. Unless you are fortunate enough to live in the narrow band of cool coastal weather. Excess heat can cause the stress reaction you see.
Hard water can also cause the reaction you see. You have the problem that you do not get enough rain to make collecting rain water to water your trees a feasible project. I get rain pretty much at least once every week. Rare do I have to go more than 2 weeks between rain storms. So forget about collecting rain water.
Here I am going to get "out over my ski's" in that I don't have a water problem. If you have a local bonsai club, you need to talk to those who have healthy looking maples and talk with them about how they solved the bad water issue. I'm going to propose a few fixes, but locals who have already dealt with the problem will have a better handle on it.
The problem with the water is not really pH. The pH is used as a "stand in" for the real issue, high mineral content. Rather than mess around trying to lower pH, better would be to actually try to remove some minerals from your water. Get a hold of your municipal water report, by law, cities are required to publish this information at least once a year. If the total dissolved solids are more than 600 parts per million, you have a mineral content that pH adjustments will never make acceptable for maples.
Rather I would purchase a reverse osmosis (RO) system. There are small and medium size systems that can make a gallon per hour, of chemically pure water with no minerals. Distilled water is also chemically pure. Bottled water for human drinking, can be good, or it can be bad, as minerals are often added to give the water better flavor. If the purchased bottled water for human drinking is less than 300 ppm total dissolved solids, it is "good enough" for maples. But the information will be difficult to get and will vary from brand to brand, and possibly from bottling plant location to bottling plant location. For example Aqua-Fina has many bottling plants, some locations might be fine, other locations might be too high in minerals to keep maples happy.
I would go with an RO system. An begin watering your maple with RO water. Add about 5 % to 10 % of your tap water back to the RO water, as you need a few minerals in the water to avoid leaching out all the essential minerals, like N-P-K, needed to keep the maple healthy. Or instead of back adding some tap water, to the pure RO water add about 1/8 teaspoon per gallon of your normal fertilizer. That little bit will bring the Total Dissolved Solids up to about 40 ppm TDS which will prevent tissues from being leached out.
That would be my approach. But like I said, I don't have this problem. My water is only 225 ppm TDS coming out of the tap. Check with other San Diego maple growers and find out what they do.
